Small on-premises switching systems provide service to one or more central office lines and a modest number of standard telephone station sets and other subscriber facilities e.g., answering sets, FAX machines, personal computers, etc. The control facilities for such systems range from a full set of control apparatus, which are permanently assigned to the central office lines on a one-for-one basis, to common control apparatus which is shared among all central office lines and all subscriber facilities. In common control systems, facilities to detect: ringing and on-hook/off-hook supervision are associated with the central office lines and with the station sets to recognize requests for service e.g., call origination, call termination, call transfer, etc. In such systems, the common control apparatus serves such requests.
While common control systems permit the addition of central office lines and station apparatus to systems, the initial cost of the common control apparatus generally precludes their use with only a few central office lines. On the other hand, in systems which employ control apparatus dedicated to a single central office line, the control apparatus is generally under utilized.
Although present on-premises switching systems provide a wide range of subscriber services, there is no provision for sharing the incoming and outgoing calling loads among bi-directional central office lines.